One who, in an argument, takes a position, and when that position is destroyed by evidence and well-reasoned argument, acts as if the reasoned argument never happened, instead sticking to his original position while resorting to logical fallacies such as bad math, unwarranted assumptions, appeals to emotion, wishful thinking, false and deliberately misleading data and analysis, or ad-hominem attacks on his opponents. One who argues in bad faith.

Avik Roy exhibits derpitude. Search New Republic. Search also Krugman blog bad-faith-and-civility-health-care-edition: "If someone says the sky is green, you prove that it’s actually blue, and the next day he comes back once again insisting that the sky is green, and this happens repeatedly, you eventually have to acknowledge that mannerly debate about the color of the sky just isn’t enough; you have to go meta, and talk about the fact that this guy and his friends just aren’t in the business of honest discussion."

A word that can mean many different things; usually negative. It can be a noun, verb and adjective. Synonyms include stupid, retarded, idiotic, idiot, dumb etc...

Variations of the word include derpy, derped, derpette, derpity der hurrrr. Herp is also related to derp and usually follows derp in a derpy interjection. Herping is almost always a verb, and when in combination with derp it does not have to be negative.

Noun: "Wow, that guy is a derp"
Verb: "Wow, that guy just derped everything up"
Adj: "Wow, that guy is derpy..."